In-line heating is a thermal treatment used for pumpable products. In-line heating is intended to mean heating while the product to be heated is transported and is not stationary as e.g. in a container. In-line heating is especially applied in the field of food products. In-line heating may provide a means of pasteurizing or sterilizing a food product in a relatively fast and efficient way. It is in particular advantageous for pasteurizing or sterilizing of food products in which taste and/or texture may become affected upon extended heating. Examples of this kind of food products are soups and sauces comprising a liquid base and, possibly, pieces (e.g. meat, fish or vegetable). The quality of both the liquid base and the pieces may be adversely affected by conventional heating processes, in terms of for example taste and/or texture. Solid meat and/or vegetable pieces may contribute to a pleasant bite or mouthfeel in the soup and/or sauce. It may be appreciated by the consumer that organoleptic features like colour and taste are not affected by overcooking, and are preserved during the pasteurization or sterilisation process.
In-line heating devices to conduct this process are known in the art. They comprise closed flow passages where energy is transferred to the product by a process of convection and conduction, or directly deposited into the product by volumetric heat transfer means. This may involve energy transfer from a hot medium such as pressurized water or condensing steam, or it may concern volumetric energy deposition by volumetric means, examples being Ohmic, microwave or Radio Frequent heating.
Conventionally, the food product is pumped through the flow-passage at an approximately constant rate. A transport mechanism, like for example a pump, may be situated at the entrance of the heating tube and propels the food product through it.
A problem observed in the conventional process of in-line heating using an approximately constant flow rate is that most pumpable food products inside the flow passage exhibit marked residence time distributions. For Newtonian fluids in the laminar flow regime, for example this maximum velocity is double the average velocity of the stream. In general, any pumpable food product, either laminar or turbulent, has a velocity profile in tube flow and hence a residence time distribution is also present.
In conventional in-line heating systems each annular layer of product is therefore characterised by a different velocity and hence a different residence time in the flow passage and consequently by a different increase of temperature. The layer close to the wall of the flow passage may become overcooked compared to the relatively central layers. This problem becomes especially relevant when solid food particles are present in the pumpable food product, such as e.g. pieces of vegetable, and the organoleptic properties like taste and mouth feel deteriorate when they become overcooked.
To guarantee sufficient pasteurization or sterilisation of the food product in a conventional in-line heating process, the velocity used for the thermal calculation, is adjusted to the fraction in the tube which has the highest velocity at the cost of the quality of the slower moving parts. When adjusting the speed to the slower moving parts, to maintain for example their desired organoleptic features, the system doesn't enable a correct pasteurization or sterilisation process of the fraction with the highest velocity.
Accordingly, the aim of the present invention is to provide a process comprising in-line heating for a pumpable food product operated in such a fashion that overcooking of at least part of the food product is reduced, preferably, wherein overcooking of at least part of the food product does not occur. Preferably, such a process is easy to conduct using conventional flow passages in the heating device, allows easy cleaning of the flow passage and provides high process reliability. Preferably, the process operation is especially suitable for the pasteurization or sterilization of pumpable food products comprising solid particles.